The present invention is related in general to the field of semiconductor devices and electronic systems and more specifically to an automated system for accurate ball feature measurements in computer-controlled bonding machines used in integrated circuit assembly.
In integrated circuit (IC) assembly, an IC chip is typically mounted on a leadframe and electrically connected to it by metallic segments. Commonly, the chip assembly is encapsulated in a protective package (for instance, ceramic package, or plastic package using molding process). Typically, the IC chip has a plurality of bond pads, which are often positioned around the chip perimeter; these bond pads have predetermined bonding area and spacing (bond pad pitch). The leadframe usually has a plurality of narrow xe2x80x9cinnerxe2x80x9d leads for attachment to the segments and inclusion in the package, and a plurality of wider xe2x80x9couterxe2x80x9d leads for attachment to other parts such as solder attachment to circuit boards.
The metallic segments used for electrical connection of the IC chip to the leadframe include wires and ribbons, and are attached by ball bonding, stitch bonding, or wedge bonding techniques. Wire bonding is a process in which a wire may be welded from a chip bond pad to the tip of an inner lead of the leadframe. As an example, in wire ball bonding the ball is attached to the chip bond pad and the stitch to the leadframe inner lead. For a given device type, there is a set of locations expressed in x and y coordinates which defines the bond locations on the chip and on the lead tips. These locations are generally stored collectively in a computer file, sometimes referred to as xe2x80x9cDevice Programxe2x80x9d. Apart from the bond head, capable of providing x-y-z motion needed for bonding, a wire bonder has a material handling subsystem and the vision subsystem.
Conventional semiconductor computerized wire bonders use x-y tables to move the bonding capillary over the device for bonding between the chip and the leadframe. The x-y coordinate tables are driven by complex electrical and mechanical components that may convert rotary and linear motions of the axis drive motors to create the needed positioning. The bond head also carries several other components such as the z-axis drive motor, a camera and optics for vision functions, and further components required to control wire bonding. Specific features of the capillary and its alignment are described in a number of U.S. patents and patent applications. Examples are: U.S. Pat. No. 5,934,543, issued on Aug. 10, 1999 (Koduri et al., xe2x80x9cWire Bonding Capillary having Alignment Featuresxe2x80x9d); and application Ser. No. 08/993,638, filed on Dec. 18, 1997 (Koduri, xe2x80x9cWire Bonding with Capillary Realignmentxe2x80x9d). The interaction of capillary and vision system is illustrated, for example, by U.S. patent application Ser. Nos. 09/191,812, filed on Nov. 13, 1998 (Koduri et al., xe2x80x9cAutomation of Optics Offset Measurement on Wire Bondersxe2x80x9d); Ser. No. 09/111,642, filed on Jul. 08, 1998 (Koduri et al., xe2x80x9cAn Efficient Hybrid Illuminatorxe2x80x9d); Ser. No. 09/111,977, filed on Jul. 08, 1998 (Koduri et al., xe2x80x9cAn Efficient Illumination System for Wire Bondersxe2x80x9d).
When a unit is indexed in by the material handling system for bonding, the position of the leadframe and the chip is not always the same because of variations in the handling and previous manufacturing (such as variable chip positioning during attachment to the leadframe). Without knowing accurately the target bonding locations, the bond head cannot place the bonds as expected. To aid this process, a machine vision system is employed. A typical vision system consists of a set of optics to provide the needed illumination and magnification of the device, a camera to capture the image provided by the optics and an image processing system to store and analyze the captured image.
Before bonding a device, it is essential to determine the device program with all the coordinate locations of the bonds that need to be created. With respect to a predetermined set of reference locations, those locations are often referred to as xe2x80x9chomesxe2x80x9d. A typical device may have one or more xe2x80x9chomesxe2x80x9d. Generally, the identification of homes needs to be done individually for each device to be bonded. It is common practice to use a three-step process to enable such identification.
In the step of xe2x80x9cteachingxe2x80x9d, the coordinate locations of the homes and all the needed bonds are identified and saved to create the xe2x80x9cdevice programxe2x80x9d. Once generated, a device program can be stored, copied and/or shared between multiple machines as needed.
In the step of xe2x80x9cregenerationxe2x80x9d or xe2x80x9cretrievingxe2x80x9d, a human operator helps in locating the homes of the first device after loading in the information from the previously saved device program. At this point, the machine captures and saves a set of images, called xe2x80x9creference imagesxe2x80x9d or xe2x80x9creferencesxe2x80x9d in the neighborhood of each home.
In the step of xe2x80x9cbondingxe2x80x9d, the machine indexes one unit or more at a time into the workstation under the bond head. At this point, the vision system, with the aid of a pattern recognition system, attempts to relocate the matching locations with the saved references. After finding the new coordinates of the matching references, the home and bond locations are re-computed for that specific unit from the device program data. The process of relocating the references and homes is normally referred to as xe2x80x9caligningxe2x80x9d the device. Using the specific bond locations, the device can now be bonded. The process of indexing, aligning and bonding is repeated without any human intervention as long as nothing abnormal happens on the machine.
Problems in wire bonding techniques arise in part from the technology trends to increase the number of leads in a given package and to make IC chip packages smaller. As consequences, the bonding pads located on the chip receive smaller areas and are spaced closer together, and the inner leads of leadframes are made narrower and closer together. These trends demand tighter control of wire bond ball and stitch geometries and placements. For instance, even small bond placement errors may result in device loss.
For the bond machines, errors in x-y tables and motors need to be reduced. At the microscopic level, the shape of free air balls and the precision of forming nail-head xe2x80x9cballxe2x80x9d-attachments need to be controlled such that uniform reproducibility can be guaranteed. These controls for forming and attaching wire xe2x80x9cballsxe2x80x9d determine to a great extent the quality of the bonding process.
The emerging technical problems for automated bond machines can be summarized as follows:
Accuracy: Small ball/fine pitch bonding requires a very accurate system to be able to place the ball completely on the bond pad. The current systems have difficulties in achieving this.
Large variations in illumination settings can lead to variations of the images as seen by the optics and the camera.
The current systems cannot handle x-y table inconsistencies. For small pad/fine pitch bonding, a small error in ball placement can cause the ball to be partially off the pad.
Human error during regeneration of alignment program: Ball placement is greatly affected by the accuracy of the alignment program. There are many steps to this regeneration process and thus there are many chances for human error.
Time spent performing alignment regeneration: Whenever a device is to be bonded, a human operator typically has to spend a finite amount of time to perform an alignment regeneration.
U.S. patent application Nos. 60/201,910, filed on May 04, 2000 (Koduri et al., xe2x80x9cSystem and Method to Reduce Bond Program Errors of Integrated Circuit Bondersxe2x80x9d), No. 60/204,997, filed on May 16, 2000 (Bon et al., xe2x80x9cSystem and Method to Customize Bond Programs Compensating Integrated Circuit Bonder Variabilityxe2x80x9d), and No. 60/206,493, filed on May 23, 2000 (Koduri et al., xe2x80x9cSystem and Method to Recreate Illumination Conditions on Integrated Circuit Bondersxe2x80x9d) describe methods how a network of relationships between reference images, bond locations, and illumination conditions can be used to adaptively compensate for variable characteristics of a slave circuit compared to a master circuit, and a slave bonder compared to a master bonder. The present invention is related to these patent applications.
Unfortunately, however, they do not address methods how an individual bonder can correct gradual and systematic errors, or random and unpredictable errors in the bond pads or the bonding operation. In U.S. patent application No. 60/222,529, filed on Aug. 02, 2000, (Koduri, xe2x80x9cMethod of Self-Correcting Bond Placement Errors of Integrated Circuit Bondersxe2x80x9d), the error between the intended bonding location and the actual location is determined by computing distances and directions between the pre-programmed and actual pad centers and the actual bond location. The present invention is related to this patent application. However, no method is described to compensate for variations of xe2x80x9cballxe2x80x9d bond featuresxe2x80x94a key ingredient of bond quality control. U.S. Pat. No. 5,991,436, issued on Nov. 23, 1999 (Koljonen et al., xe2x80x9cApparatus and Method for Inspecting Wire Bonds on Leadsxe2x80x9d), concerns itself only with the control of wire stitch bonds on metallic leadframes.
An urgent need has therefore arisen for a fast, reliable and flexible system and method to inspect, measure and correct xe2x80x9cballxe2x80x9d-shaped wire bonds for controlling bond process quality. The system and method should be flexible enough to be applied for different IC product families with a wide spectrum of design variations, and for different bond machines. The system and method should spearhead solutions toward the goals of improved product yield and reliability, preferably without investment in new equipment.
The present invention provides a computerized system and method for inspecting and measuring a ball-shaped wire bond formed by an automated bonder pre-programmed to attach a connecting bond onto a bond pad of an integrated circuit by first obtaining a first image of said bond pad before bond attachment, then determining the coordinates of the center of said pad. Second, the bonder is instructed to attach a ball-shaped wire bond to the center of said pad. Next, a second image of said bond pad is obtained after bond attachment; this second image comprises an image of the ball-shaped portion of the bond and an image of the wire portion of said bond. The coordinates of the center of the ball-shaped portion of the bond are obtained by computer processing of the first and second images. The coordinates of the bond center and the pad center are compared, creating information for quality control of the bonder instruction and the bonding process. Finally, the bond process quality is controlled by inputting new bonder instruction for correcting any identified differences between the center coordinates.
The present invention is related to high density ICs, especially those having high numbers of input/outputs and tight constraints in package outline and profile. These ICs can be found in many semiconductor device families such as processors, standard linear and logic products, digital and analog devices, high frequency and high power devices, and both large and small area chip categories. Since the invention aims at designing devices with minimum geometries and high reliability, it supports continually shrinking applications such as cellular communications, pagers, hard disk drives, laptop computers and medical instrumentation.
It is an aspect of the present invention to provide an automated system and method for inspecting and measuring wire bond features needed for controlling bond process quality. It is another aspect of the present invention to provide a highly flexible system and method, applicable not only to semiconductor wire bonders, but also to many automated machines pre-programmed to perform mechanical action onto action sites of objects. These aspects are achieved by the embodiments of an xe2x80x9cimage comparatorxe2x80x9d subsystem of the invention:
Two images obtained by a vision system are processed in an automated method by first extracting the image of the mechanical result of the action, in the example of wire bonding the attached bond itself. The extracted image is then filtered by removing noise, and the filtered image is reduced to contours only, outlining the geometrical shape of the bond portion having the ball-shaped bond. The geometrical outlines are further reduced to thin, narrowly defined contours. The contours of the ball-shaped bond portion are then approximated by a simplified search model selected for best fit into the contour outline, and the coordinates of the center of the search model are determined. Examples of search models include circles, ellipses, polygons, and synthetic images.
Another aspect of the invention is to achieve the improved quality control by automatically computing a new bond center coincident with the pad center and implementing the new center into the bonder as the corrected bond attachment instruction.
Another aspect of the present invention is to introduce methods of quality control which are flexible so that they can be applied to many families of electronic structuresxe2x80x94reaching from semiconductor chips to piece parts, such as leadframes and interconnectors, to device packages, to electronic substrates, and to whole assemblies on motherboardsxe2x80x94and are general so that they can be applied to several generations of products.
Beyond the electronics realm, the computerized system and method of this invention can be generally applied to control quality by inspecting mechanical features produced by machines prepared to mechanically work on action sites of objects. The automated system for accurate placement continuously inspects and measures shapes, of connections, and corrects any deviation in the placement process, which requires high precision on equipment. Besides wire bonding, examples are local actions such as hole drilling, nail attaching, and mechanical bridging. A machine vision system in conjunction with a robotic placement mechanism mimics the human hand-eye coordination. The stringent requirements placed on the components can than be relaxed as any deviation in performance is going to be automatically corrected for.
The technical advances represented by the invention, as well as the objects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.